Steel melts, prior to casting into ingots and rolling, are commonly subjected to a deoxidation and desulfurization treatment which may be supplemented by or can, in part, involve the treatment of the steel melt with calcium-containing treating agents capable of purifying the melt and reducing the sulfur content thereof.
The calcium-containing treatment agent may be finely divided (fine grain) calcium, calcium compounds such as calcium carbides, and calcium alloys or compounds such as calcium-silicon which contains 30% by weight calcium, 60% silicon and 10% iron, or the like. Other treatment agents which may be used can contain, in addition to silicon, elements such as aluminum and manganese. The carrier gas should be a neutral or inert substance such as argon.
In conventional processes the treatment of the melt with calcium-containing agents is generally carried out with a constant equilibrium between supply of the agent and consumption thereof by the purification of action. The thermodynamic and reaction-kinetic parameters of the system determine the maximum rate at which the calcium-containing treatment agent is capable of reaction. The above-mentioned equilibrium is achieved when this rate is equaled by the rate at which the calcium-containing agent is supplied to the melt. Whatever calcium-containing agent is supplied to the melt, therefore, is immediately reacted therewith.
This system has the advantage over still older processes, in which the calcium-containing treatment agent in a predetermined quantity (for example an amount of 2 or more kg/ton of the melt) is introduced in a single step and all together into the melt. In the latter case some of the calcium-containing agent or the calcium thereof is evaporated without having undergone reaction with the melt and hence the equilibrium process manifests a saving of the treatment agent. Desulfurization is also improved by the equilibrium method.
However, the ductility characteristics of the manufactured steel, measured in terms of the break contraction, is so high as to be undesirable and it has been found that the isotropy of the ductility characteristics of the steel requires improvement. For the purpose of the present application, the term "break contraction" will be used to refer to the contraction of a dimension of the manufactured steel body under tension at rupture and is measured by the relationship ##EQU1## and is given in percent; d' is the linear dimension at break, d is the corresponding dimension prior to the application of tensile stress to the body. An increased percentage value of the break contraction corresponds to improved ductility.
In the aforementioned applications, we generally describe an improved system for treating a steel melt with calcium or calcium-containing materials.